Starts out quick, gets slow

I'm not sure if anyone can point me at where I should be looking for a solution, but I'm hoping

When I power my router on, using the Speed test at http://speedcheck.ispconnect.co.uk/, I generally get between 3Mbps and 4.5Mbps. This ties back to what my router is connected at (rougly 3679 when the speedcheck is showing 3Mbps).

However, over the next few minutes if I re-run this test, the speed gets slower and slower. Normally around 10 minutes after power on, I am seeing speeds of less than 1Mbps.

I've replaced the microfilter, the DSL cable and even the router (although the new one is temporarily out of action as I can't work out how to get it to be a router not a weird nat firewalling device).

Any advice would be much appreciated.